Method and apparatus for the continuous doping of semiconductor materials



Oct. 21, 1969 SHENG EI'AL 3,473,510

METHOD AND APPARATUS FOR THE CONTINUOUS DOPING OF SEMICONDUCTOR MATERIALS Filed Feb. 25, 1956 INVENTORS HENRY P. SHENG V ETHOMAS WOOTEN A a, @M,

BY m 24; i 7- c ATTORNEYS United States Patent US. "Cl. 118-495 4 Claims ABSTRACT OF THE DISCLOSURE Apparatus and a method for continuously doping semiconductor materials whereby substantially independent chambers are passed in series through an elongated furnace wherein semiconductor materials placed in the chambers are sequentially exposed to preheating, doping and cooling zones in the furnace.

The present invention relates to a continuous method for doping semiconductor materials and to an apparatus for carrying out the process.

For the most part, the doping of semiconductor materials is currently carried out by batch processes. Such processes generally comprise placing a body of semiconductor material in a crucible or other suitable vessel, pushing the vessel into a furnace and introducing a suitable doping agent into the furnace. After doping is completed, the vessel is then withdrawn and the semiconductor material removed.

The batch technique makes it very difiicult to obtain a high yield of usable material due to the unreproducibility of the product. For example, the temperature of the boat pushed into the furnace may effect a temperature variation of several degrees and this in turn influences the extent of the doping. Also, the length of exposure of the semiconductor material to the doping atmosphere is difficult to control under such circumstances and numerous human errors intrude.

The primary object of the present invention is to provide a continuous method and apparatus for doping semiconductor materials which enables a high yield of usable doped semiconductor material to be obtained.

The invention generally comprises passing a series of substantially independent chambers through an elongated furnace. Semiconductor material placed in the chambers is sequentially exposed to preheating, doping, and cooling zones in the furnace. Each of the treating stations within the furnace is maintained substantially independent of the others and the temperature profile, drive speed through the furnace, and atmosphere at each treating station are all carefully controlled so as to insure reproducible results.

The invention will be more fully appreciated in the light of the following detailed description of a preferred embodiment of the invention and the best method and apparatus which has been contemplated for carrying out the invention. An appreciation of the invention will be further assisted by reference to the accompanying drawing, in which:

FIGURE 1 is a side, elevational view, partly in crosssection, of an apparatus for carrying out the invention,

FIGURE 2 is a top, cross-sectional view of the furnace shown in FIGURE 1 of the drawing,

FIGURE 3 is an end view of the unload end of the furnace shown in FIGURE 1 of the drawing, and a somewhat schematic view of ancillary apparatus for introducing the doping atmosphere to the furnace,

FIGURE 4 is a detailed perspective view of a preferred embodiment of a vessel for transporting the semiconductor material through the process and apparatus of the invention, and

FIGURE 5 is a detailed perspective View of another vessel for transporting the semiconductor material through the process and apparatus of the invention.

Referring to the accompanying drawing and in particular to FIGURE 1, it will be seen that one embodiment of apparatus in accordance with the present invention comprises a furnace having an internal hollow core along its longitudinal axis. In the portion of the furnace shown in section, it will be seen that the furnace comprises an outer housing 11 lined with a suitable insulating material 12. A sleeve member 13 which may be of generally cylindrical configuration is located in the core of the furnace and extends somewhat beyond each end of the furnace. Mounted in a compartment between sleeve 13, enclosure 11 and insulating material 12 are a series of heating elements 70-73 by which the internal temperature of the furnace is controlled. These heating elements are preferably independent one from the other so that local control of the internal temperature of the furnace may be exercised.

Insulating members 14 are positioned at each end of the furnace surrounding the sleeve.

Positioned along the length of sleeve 13 and extending beyond each end of the sleeve is a D tube 15. The back of the D tube is in a horizontal plane and provides a flat vfloor within sleeve 13.

A series or train of boats or vessels 50 are positioned within the core of the furnace in sleeve 13 and supported by D tube 15. The train of boats is driven through the furnace by ram 17 which in turn is driven by motor 18 through gear 19 and rack 20. The rack 20 and ram 17 may be supported by standard 21 to align them with the last boat in the train.

While the drive means for advancing the train of boats or vessels through the furnace has been shown as a particular variable speed motor and ram arrangement, it will be understood that any suitable drive means for pushing the boats through the furnace at a desired speed may be utilized.

A series of conduits -34 inclusive are shown exiting from the side of the furnace and communicating through valves 37 inclusive with exhaust manifold 40. Each conduit 30-34 communicates with a restricted zone or chamber within the furnace.

Referring briefly to FIGURE 4 of the drawing, the substantially independent chambers within the furnace are preferably formed by the configuration of the boats themselves. In a preferred embodiment, the boat preferably comprises a tray or support element 51, in which the semiconductor material to be doped is placed. A back plate 52 is attached to the tray element 51 and has a configuration conforming closely to that of the open section of the furnace. Thus, a series of such boats or vessels in end to end relationship form a series of substantially independent chambers within the core of the furnace.

Alternatively, the D tube 15 may be eliminated and boats of the configuration shown in FIGURE 5 may be employed. In this case, the tray element 56 of boat is in the form of a D tube. The semiconductor material is supported on the flat, horizontal surface 57 of element 56. Back plate 58 is circular and provides an effective barrier between the successive vessels driven through sleeve 13.

Referring now to FIGURE 2 of the drawing which is a top sectional view of the furnace portion of the apparatus shown in FIGURE 1, it will be seen that semiconductor Wafers or discs 67 are placed in the tray portion 51 of boats 50. Each back plate 52 of the boats closely but slidingly engages the upper arch of sleeve 13 and the floor provided by the back of D tube 15. This establishes a series of chambers or compartments within the furnace, each compartment being approximately the length of an individual boat.

Conduits 30-34 inclusive are shown extending through the side wall of the furnace and communicating with the series of chambers formed within the furnace by the train of boats. As was noted in connection with FIG- URE 1, this series of conduits is connected through suitable valves with an exhaust manifold 40. On the other side of the furnace, conduits 60-64 inclusive also communicate with the chambers formed within the furnace. These conduits provide means for introducing a desired atmosphere into the chambers which are evacuated through exhaust conduits 30-34 respectively.

Conduits 60, 61, 63 and 64 are preferably supplied through valves 65 and 66 with a suitable inert gas, such as nitrogen, or a mixture of nitrogen with a small amount of oxygen to maintain a substantially non-reactive atmosphere within the respective chambers during the preheating, and cooling stages. A suitable doping atmosphere is introduced through conduit 62 and evacuated through conduit 32.

As illustrated in FIGURES l and 2, the train of boats 50 containing semiconductor material to be continuously doped in furnace is driven from left to right, so that the left-hand end of the furnace may be referred to as the feed end and the right-hand side as the unload end. In the preferred embodiment, the first stage of the treatment to which the semiconductor is subjected within the furnace is a preheating step. Heat for this operation may be provided by heating coils 70 and 71 and the atmosphere is controlled by introducing gases through conduits 60 and 61 and evacuating through conduits 30 and 31, respectively. The heating elements are preferably in the form of coils completely surrounding sleeve 13.

Since gaskets or coils of insulating material 14 are positioned at each end of the furnace and since there is a certain amount of heat loss through the ends of the furnace, the temperature profile gradually rises from the ends towards the center. In addition, by providing independent controls for the heating elements, the temperature profile within the furnace may be subjected to further monitoring.

Referring now to FIGURE 3 of the drawing, which is an end view of the unload end of furnace 10 and ancillary apparatus for supplying the doping atmosphere to the furnace, it will be seen that each boat 50 is positioned in sleeve 13 so that boat back 52 substantially seals off tray element 51 from following boats. The bottom of tray 51 and boat back 52 rest on and slide along the back of D tube which rests in the lower half of sleeve 13. Inert gas input and output conduits 64 and 34 are shown communicating with the interior of the furnace by dotted lines. Doping atmosphere input conduit 62, partly obscured by conduit 64, is shown with its associated apparatus for generating the desired doping atmosphere. l

The ancillary apparatus used for supplying dopant to the furnace is similar to that conventionally used in batch processes and generally comprises a source of oxygen or other carrier gas not shown, which is supplied through conduit 80. The flow of the gas is controlled by valve 81 which communicates with filter 82 and cold trap 83. Together, filter 82 and cold trap 83 combine to dry and purify the carrier gas. The gas is then conducted through conduit 84 to conduit 62 for introduction into the furnace. Part of the input stream of carrier gas from conduit 80 is conducted through conduit 85 and valve 86 and thence through filter 87. The carrier gas from filter 87 is then passed through doping agent source 88 which may contain a body of phosphorus oxychloride (POCl or other suitable source of doping agent. The carrier gas containing doping agent is then conducted through conduit 89 and cold trap 90 and is combined in conduit 62 with additional carrier gas prior to introduction into the fumace. Suitable flow meters and valves are inserted in the system shown to control the rate of flow of the carrier gas over the dopant source and to proportion the amount of dopant picked up by the carrier.

In a particular system, the furnace shown may be approximately 36 inches in length with about 6 inches between the input and output conduits, 60-64 and 30-34 respectively. The internal diameter of sleeve 13 may be about 3 inches. The boats 50 may be about 6 inches in length and may be made of alumina with a semi-circular alumina disk bonded to a tray section to provide backing 52. Sleeve 13 may have a length of about 44 inches.

In operation, the heating elements 70-73 are actuated to provide a peak internal temperature at the center of the furnace in the range of from 800 to 1300 C. The first of a series of boats 50 are placed on the extension of D tube 15 and additional boats are arranged behind in end-to-end relationship. Additional support means may be necessary outside the furnace to support a particularly long train of boats or the D tube may simply be extended to provide the necessary support. Suitable drive means are then actuated to drive the train of boats through the furnace at a rate of from about to 22 inches per minute. At the preheat end of the furnace. the temperature is gradually raised to about 800 C., and an inert gas, preferably nitrogen containing two or three percent of oxygen, is continuously flowed through the chambers at the preheat end of the furnace. In the middle of the furnace, a peak temperature is achieved, preferably around 1000" C., and a gas containing a doping agent is introduced through conduit 62 and withdrawn through conduit 32. Finally, in the cooling end of furnace, the temperature is reduced gradually back down to about 300 C. before the boats exit from the furnace. Again, nitrogen containing a small amount of oxygen or other inert gas is flowed through conduits 63 and 64 and withdrawn through conduits 33 and 34 to provide the atmosphere in the cooling end of the furnace. In this manner, continuous doping of semiconductors may be carried out with a very high yield of useful product. This is accomplished by forming separate chambers within the furnace by means of the series of semiconductorcontaining boats driven through the furnace at a con stant speed. The preheating in nitrogen enables the temperature in the doping zone to be closely controlled. Separate chambers are further ensured by creating a slight vacuum in the exhaust lines.

It will be obvious that the apparatus may be readily modified for operation using boats of the configuration shown in FIGURE 5.

EXAMPLE In a preferred embodiment of the invention, semiconductor wafers are placed in a series of boats and advanced through the furnace at a rate of .4 inch per minute. The peak temperature at the center or doping station of the furnace is maintained at 860 C. A mixture of dry nitrogen and oxygen in the ratio of 400 cc. per minute of nitrogen to 15 cc. per minute of oxygen is introduced through each of conduits 60, 61, 63 and 64. A doping atmosphere is provided in the center chamber of the furnace by flowing through the center chamber within the furnace a mixture of 425 cc. per minute of oxygen and 40 cc. per minute of oxygen which has been passed over phosphorus oxychloride source material. In this manner, close control of semiconductor doping is readily obtained and reproducible results are achieved.

It will be obvious to those skilled in the art that various modifications may be made in the process and apparatus illustratively described herein without departing from the spirit or scope of the invention as expressed in the following claims.

What is claimed is:

1. Apparatus for continuously doping semiconductor materials comprising a furnace having an elongated central core, a plurality of vessels containing semiconductor material to be doped, means for substantially isolating each vessel from the others within said core thus forming a plurality of substantially independent chambers, means for driving said plurality of vessels through said furnace, a plurality of inert gas carrying inlet and outlet conduits passing through said furnace and communicating with said core and at least one dopant gas carrying inlet and outlet conduit passing through said furnace and communicating with said core, said plurality of conduits being spaced along the length of said core so that as each of said plurality of vessels is driven through said core, each independent chamber is sequentially brought into and out of juxtaposition with each of said plurality of inlet and outlet conduits thereby sequentially exposing and doping the semiconductor material contained within said vessels to the gas carried within said conduits,

2. The apparatus of claim 1 wherein said means for isolating said vessels comprises a plate-like barrier attached to each of said vessels, said barrier having a configuration corresponding to the radial cross-section of said core and slidingly engaged therewith.

3. The apparatus of claim 1 further comprising means for cooling said semiconductor material after said doping.

4. The apparatus of claim 2 wherein said vessels having said attached barriers are positioned within said core in end-to-end relationship such that said independent chambers are formed between the walls of said core, the walls of each of said vessels and the barrier is attached to said adjacent vessels.

References Cited UNITED STATES PATENTS 1,465,071 2/1968 Baker 34-216 2,873,222 2/1959 Derick et al. 148-189 X 3,279,964 10/1966 Beck 148-189 3,298,112 1/1967 Stokes 34-217 L. DEWAYNE RUTLEDGE, Primary Examiner R. A. LESTER, Assistant Examiner US. Cl. X.R. 

